1. Field of the Invention
The present invention relates to a piston engine, in particular in a motor vehicle.
2. Description of the Related Art
A piston engine usually has several cylinders, their combustion chambers being connected at the input end to a fresh gas system and at the output end to an exhaust system. The fresh gas system for each cylinder usually comprises a separate gas path for each over which fresh gas can be supplied to the respective combustion chamber. The fresh gas system is designed with dimensions so that a fresh gas stream can be supplied to the cylinders as is needed in full-load operation of the piston engine. At partial load, the quantity of fresh gas supplied to the combustion chambers must be reduced. It is customary here to throttle the fresh gas system, i.e., provide it with an adjustable throttle mechanism, in particular a throttle valve with which the fresh gas stream supplied to the individual gas paths can be throttled as needed. One disadvantage of throttle fresh gas systems is that the respective throttle mechanism used also forms a flow resistance at full load and allows only relatively small flow velocities at partial load. Fluid-dynamic effects at low flow rates in the fresh gas stream cannot be utilized or can be utilized only inadequately in charging the combustion chambers. Such fluid-dynamic effects, e.g., spiral flow and tumble flow can be utilized to improve the combustion process in modern piston engines in order to reduce fuel consumption and pollution emissions. Since piston engines are operated mostly at partial load and only comparatively rarely at full load, the greatest potential for reducing fuel consumption and pollution emissions is in the partial load range. Therefore there is the desire to utilize fluid-dynamic effects in the partial load range of operation as well to reduce fuel consumption and pollution emissions.
Pischinger, Variable Valve Control II, Expert Publishers, pages 244 through 260 states that the fresh gas system in a piston engine should be dethrottled and/or designed to be throttle-free upstream from intake valves, i.e., the fresh gas system does not contain any special throttle mechanism for throttling the fresh gas stream. For implementation of the fresh gas charging, which depends on the particular load state of the piston engine, there is a known variable valve control which makes it possible to adapt the opening times and closing times of the intake valves in a suitable manner. During the intake stroke of the respective piston, the variable valve control may also implement two charging phases with an interval of time between them. It is likewise possible to vary the opening stroke of the intake valves. To implement these versatile possibilities of variation, the known piston engine works with an electromagnetic valve control that makes it possible to arbitrarily open and close the intake valves and arbitrarily alter their opening stroke. However, such electromagnetic valve controls are much more expensive than traditional valve drives controlled by camshaft.